(1) Field of the Invention
The present invention relates to a light beam deflection scanner for use in an electrophotographic recording apparatus.
(2) Description of the Prior Art
Conventionally, there have been electrophotographic recording apparatus in which a light beam modulated based on a image data stream is scanned in the main scan direction to write a static latent image on the photosensitive member surface so that the static latent image is developed into a visual image as a recorded representation. Examples of marketable products in which a recording apparatus is mounted are such digital copiers, laser printers, laser facsimile machines and the like.
Recently on the market, a more speedy recording process has been desired for recording apparatus, so that investigation into speeding up of the electrophotographic process has been continued. Among the apparatus using a laser scanner for illuminating the photosensitive drum surface for the electrophotographic process by scanning a modulated laser beam, the rotational speed of a rotating polygon mirror as a beam deflecting device is increased or the number of mirror facets of the rotational polygon mirror is increased in order to deal with this demand.
However, in order to increase the rotational speed of a rotating polygon mirror, a large-sized mirror motor is needed, which in turn scales up the recording apparatus, conflicting with the tendency toward compactness of the product. Further, a large-sized mirror motor has an increased heating value, so that heat therefrom may cause influences upon inner peripheral devices. Moreover, use of a large-sized mirror motor may generate a large fan-like noise with the rotating polygon mirror, causing a noise problem in a silent office environment.
In recent years, a laser deflection scanner having an increased number of laser beam sources, that is, deflecting at least two laser beams by one facet of a rotating polygon mirror, has been developed and put into practice.
However, if a plural number of semiconductor laser elements are put into use, there occurs a new problem in that the light beam amount emitted from each laser element differs from other elements.
In general, a semiconductor laser element has its own output correcting circuit so as to emit a laser beam of a constant light amount for a predetermined current. However, the characteristic (beam amount) varies depending upon each individual semiconductor laser element (part).
When the light beam emitted from each semiconductor laser element is actually scanned by a laser scanning optical system, the actual amount of light irradiated on the photosensitive member can vary depending upon the distortions of the optical components.
If individual beam irradiated from the different semiconductor laser elements fluctuate as stated above, it is impossible to correctly reproduce and record fine density variations of a recorded image to be reproduced and recorded on the photosensitive member, leading to obstruction to the image reproducibility of an image having smooth gradations (density variations) such as halftone (photographic) image.
To avoid this, in the conventional configuration the light beams being scanned and illuminated on the photosensitive member is detected by a single optical sensor to adjust the amounts of the laser beams from a plural number of semiconductor laser elements so as to be equal to one another. In an actual configuration, since provision of an additional sensor for light amount correction leads to increase in cost, a synchronizing sensor (beam detecting sensor) provided for adjusting the start of scan for one end of the photosensitive member or the starting position of recording by the laser beam is commonly used for adjusting the amount of light beam irradiated from each semiconductor laser element.
The prior art references to the above technology include: Japanese Patent Application Laid-Open Hei 9 No.230259, Japanese Patent Application Laid-Open Hei 10 No.52939, Japanese Patent Application Laid-Open Hei 10 No.206763 and Japanese Patent Application Laid-Open Hei 10 No.209545.
As described in the above publications, conventionally, a plural number of light beams are detected by a single optical sensor, whereby the amounts of light beams irradiated from the individual semiconductor laser elements are corrected to be equal to each other.
However, any of the above needs a complicated correcting circuit configuration, leading to a high price of the apparatus such as a copier etc., which includes such a light beam defection scanner. So there has been earnest desire for a light beam deflection scanner capable of reliable correction with a more simple circuit configuration.
The present invention has been devised in order to solve the above problems and it is therefore an object of the present invention to provide a light beam deflection scanner with which the amounts of light beams emitted from individual elements can be made equal to each other by a simple configuration.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the invention, a light beam deflection scanner includes: a light beam generating portion having a plural number of laser beam sources to emit light beams from the laser beam sources; a deflection scanning portion for deflecting and scanning the plural light beams from the light beam generating portion; a light beam detecting portion detecting the plural light beams deflected and scanned by the deflection scanning portion to output an electric signal in accordance with the amount of light detected; and a light amount correcting portion correcting the drive voltage of each laser beam source so that the amount of light of each light beam will coincide with the others, and is characterized in that the light amount correcting portion effects a correcting process comprising the steps of: emitting a light beam with a predetermined voltage applied to one laser beam source; measuring a recovery time from when the light beam detecting portion detects the light beam until the output voltage from the light beam detecting portion returns to a predetermined level of voltage; and correcting the light beam amount based on the recovery time.
In accordance with the second aspect of the invention, the light beam deflection scanner having the above first feature is characterized in that the light amount correcting portion effects light amount correction of a light beam by applying the drive voltage determined by the previous light amount correction to the laser beam source.
In accordance with the third aspect of the invention, the light beam deflection scanner having the above first feature is characterized in that the timing of the start of irradiation from each laser beam source in the light beam generating portion is set up so that the voltage recovery time required for the output voltage from the light beam detecting portion due to a first emission from one laser beam source to return to a predetermined level will not overlap the timing at which the light beam detecting portion receives a second emission from another laser beam source.
In accordance with the fourth aspect of the invention, the light beam deflection scanner having the above first feature is characterized in that the deflection scanning portion has a rotating polygon mirror, and if the light beam detecting portion is about to receive light emitted from a second laser beam source during detection of the recovery time of the output voltage from the light beam detecting portion due to a light beam emitted previously from a first laser beam source, the light amount correcting portion briefly prohibits emission from the second laser beam source and causes the second laser beam source to emit a laser beam at such a timing as to irradiate one of the mirror facets of the rotating polygon mirror other than the due one.
In accordance with the fifth aspect of the invention, the light beam deflection scanner having the above first feature is characterized in that, when the recovery time of the output voltage from the light beam detecting portion from the previous detection of a light beam exceeds a predetermined period of time, the light amount correcting portion adjusts the output level of voltage based on which the recovery is assumed to be complete.
In accordance with the sixth aspect of the invention, the light beam deflection scanner having the above first feature is characterized in that the deflection scanning portion has a rotating polygon mirror, and the light amount correcting portion corrects the light amount of a light beam based on the mean recovery time of the output voltage from the light beam detecting portion, obtained by measuring the recovery time for each mirror facet of the rotating polygon mirror and averaging them.
According to the above first feature of the present invention, the light amount correcting portion detects the difference in amount of light between light beams from different laser beams sources, based on the difference in recovery time from when the light beam detecting portion detects a light beam until the output voltage from the detector returns to the predetermined level. Therefore, there is no need to precisely detect the output voltage at the time of beam detection, which leads to a simple configuration for determining the difference in light amount. Thus, it is possible to exactly adjust plural semiconductor laser elements to emit light beams of a substantially equal amount of light, by a simple circuit configuration.
Thus, a recording image is formed on the photosensitive member by plural number of semiconductor elements which have been corrected so as to provide almost the same amount of light, hence it is possible to correctly reproduce fine density variations and hence reproduce an image having continuous tones (density variations) such as a halftone (photographic) image, appropriately, thus providing a commercially valuable copying function.
According to the above second feature of the present invention, the light amount correcting portion effects a subsequent light amount correction by using the drive voltage determined by the previous light amount correction. Therefore, light amount correction of the light beam can be commenced from a state which is relatively close to the target light amount of the light beam, so that the adjustment can be done in a short period.
According to the above third feature of the present invention, while the light beam detecting portion is recovering from the state affected by a previous irradiation from the first laser source, it will not receive a light beam from the second semiconductor source. So it is possible to measure the exact voltage recovery time.
According to the above fourth feature of the present invention, the light beam detecting portion is positively prohibited from receiving a subsequent irradiation of light from the second laser beam source during the period in which the light beam detecting portion is recovering from the state affected by a previous irradiation from the first laser beam source, hence making it possible to detect the exact voltage recovery time.
According to the above fifth feature of the present invention, detection of the amount of light can be performed in a short recovery time by adjusting the output level of voltage based on which the recovery is determined to be complete. Therefore, it is possible to avoid the risk of the light beam detecting portion receiving a subsequent beam emitted from the second laser beam source before the output voltage from the light beam detecting portion, affected by a previous irradiation from the first laser beam source reaches the level above which the recovery is assumed to be complete. Thus, it is possible to detect exact voltage recovery time.
According to the above sixth feature of the present invention, the beams emitted from the laser beam sources can be deflected and scanned by the rotating polygon mirror in a stable manner, not being affected by the characteristics of the mirror facets, thus making it possible to eliminate variations in the recovery time due to individual mirror facets.